The present invention relates to a blow-molding method for molding a molding product by introducing a parison extruded from a die into a mold and clamping the parison, a blow-molded product obtained by the blow-molding method and a blow-molding mold used for the blow-molding method.
Conventionally, resin-made container such as a bottle or a tank has been molded by a blow-molding method where a gas is blown into an inside of a parison introduced in a mold to render shape thereof.
In the blow-molding process, cylindrical parison is generally prepared by extruding molten resin from a die and the parison is sandwiched by a mold (split die) to render shape.
Thus molded blow-molded product weighs light because of hollow construction and is superior in shock resistance and soundproofness. Further, the blow-molding can be conducted by a low pressure so that production cost can be reduced and the containers etc. manufactured by the blow-molding often has simple structure, so that the molding die can be inexpensively made because of the simple structure of the mold.
In view of the above advantages, in recent years, blow-molded product has been widely used in various industrial fields such as automobile components, housing facilities and building materials.
For instance, bumpers and spoilers in the automobile field, door panel and components attached to bath units and lavatory units in the housing facilities field, and table top and various panels in the building materials in the building material field can be cited as a typical example.
The blow-molded product is manufactured by the blow-molding generally after following four steps as shown in FIG. 6.
(a)
During a parison injection step, a cylindrical parison 99 is extruded from a mold 90 of a blow-molding machine to be introduced between a first mold 91 and a second mold 92 as the blow-molding molds.
Incidentally, a die design surface 91A is formed on the first mold 91, the die design surface 91A having a pre-contact portion 93 as a convex portion protruding toward the parison 99 and a molding portion (blow-up portion) 94 as a deeply carved concave portion.
(b)
During a mold clamping step, the first mold 91 and the second mold 92 are closed to pinch the parison 99 by land portions 91B and 92B (burr portions) located on a periphery of the blow-molding molds 91 and 92. At this time, a space between the molds 91 and 92 is an open space opening toward the outside before the molds 91 and 92 are closed. Accordingly, since a part of the parison 99 inflates toward outside of the molds 91 and 92 when the molds 91 and 92 are closed, though the parison 99 touches the pre-contact portion 93, the parison 99 does not touch the whole of the molding portion 94.
(c)
Air is blown into the parison 99 from a pin 95 during an air blowing step, so that the parison 99 touches the whole of the molding portion 94.
(d)
Subsequently, the parison 99 is cooled and solidified to obtain the blow-molded products.
The Applicant of the present application has studied and found that the appearance of the blow-molded product is impaired by streaks and wrinkles on borders of respective portions of parison in contact with the pre-contact portion 93 and the molding portion 94 and uneven transfer of grain pattern. Specifically, after touching the pre-contact portion 93 during the mold clamping step as shown in FIG. 6(b), the parison 99 touches the molding portion 94 of the first mold 91 during the air blowing step as shown FIG. 6(c). Thus, the movement of the contact portion of the parison 99 to be in contact with the die design surface becomes stagnant between the part of the parison 99 to be in contact with the pre-contact portion 93 and another part of the parison 99 to be in contact with the molding portion 94. The above-described streaks and wrinkles are generated and grain pattern was transferred uneven on account of the stagnation of the contact portion.
Conventionally, even when the streaks and wrinkles were generated on the surface of the blow-molded product, the surface was painted to hide the streaks and wrinkles or, alternatively, the grain pattern was used on the surface design to make the streaks and wrinkles less prominent.
However, since cost reduction, recycling, and solvent regulation have been required for the blow-molded products, non-coating and specular appearance have been desired.
Accordingly, various techniques has been proposed for improving appearance design not generating the streaks, wrinkles and uneven pattern transfer and for enhancing transferability to enable decoration with minute complicated patterns to achieve non-coating and mirror-finished appearance.
For instance, a molding die having a heat insulator on a mold body and a space at the back of the die design surface for introducing and discharging heated gas or cooled liquid (Japanese Patent Application Laid-Open Publication No. Hei 07-108534) and a blow-molding method where a parison is forced against a die design surface of a mold covered with a heat insulation layer by a blow gas, the method using a mold capable of accurately transferring the die design surface while keeping the parison at a high temperature (Japanese Patent Application Laid-Open Publication No. Hei 06-328549) have been proposed.
Further, a non-coating molding product having glossy smooth surface made of a thermoplastic resin having specific melt index and flexure modulus of elasticity (Japanese Patent Application Laid-Open Publication No. Hei 08-119043) and a molding product made of polypropylene having high surface glossiness (Japanese Patent Application Laid-Open Publication No. Hei 10-138324) have also been proposed as example of modifying the material of the blow-molded product.
In such technique, the streaks and wrinkles or the uneven transfer on the molding product are prevented by modifying basic molding condition such as a timing for clamping the mold or blowing the air, by controlling the mold temperature during heating and cooling, or by providing vent port or porous electroform to efficiently discharge the gas inside the mold.
However, in a multi-layer blow-molding for not coating the molding product, when a transparent material is used as a surface layer of the molding product, the streaks and wrinkles are prominently generated on the molding product even when the blow-molding mold is heated to a high temperature or when the gas inside the die is efficiently discharged.
Further, when the manufacture thereof cannot be conducted at so high temperature for the convenience of molding cycle, it is found that the above techniques cannot be used for manufacturing a molding product requiring graining for applying pattern of imitation leather or texture.
This is because, though the mold temperature generally is preferably as high as possible for applying flowability to the parison, the molding cycle can be too lengthened since much time is required for raising and cooling the mold temperature when the mold temperature is set so high, so that the productivity can be lowered as a result.
Accordingly, a blow-molding method capable of obtaining molding product having good appearance without streaks, wrinkles and uneven pattern transfer without relying on mold temperature control has been desired.
An object of the present invention is to provide a blow-molded product having good appearance, a blow-molding method for producing the molding product and a blow-molding mold therefor.
A blow-molding method according to the present invention is for molding a molding product by introducing a parison extruded from a die into a mold, the method being characterized in having the steps of: providing the mold with a parison shutter for shutting the parison to keep the parison from inflating out of the mold; opening the mold; introducing the parison into the mold; shutting the parison by the parison shutter without bringing the parison into contact with the inner surface of the mold; and closing the mold to compress the parison to bring the parison into contact with the inner surface of the mold.
In other words, a predetermined pressure difference is secured between the inside and the outside of the parison in shutting the parison by the parison shutter, so that the tension of the parison can be maintained only by the air existed in the parison at the time shutting the parison, thereby bringing the parison into contact with the whole inner surface of the mold in accordance with closing the mold.
The inner surface of the mold refers mainly to die design surface. The die design surface includes flat inner surface of the mold as well as the inner mold surface having grain pattern or complicated irregularities. The inner surface of the die corresponding to the surface seen from the outside when ejected as a molding product is usually called the die design surface.
In the present invention, the parison extruded from the die in cylindrical shape into the mold is enclosed in the space surrounded by the die design surface and the parison shutter without being inflated toward the outside of the mold by the parison shutter provided to the mold. Further, since the parison is shut by the parison shutter, the air inside the parison lost when the parison is inflated out of the mold is kept in the enclosed parison. Accordingly, the air sufficient for the parison to touch the die design surface, i.e. the air sufficient for bringing the parison into contact with the die design surface only by the air pressure inside the parison, is filled inside the parison when the mold is clamped.
Subsequently, when the mold is clamped in the condition, the parison is pressed by the mold to raise the air pressure inside the parison. Accordingly, the parison continuously touches the die design surface. In other words, the whole parison touched the mold continuously and sequentially without stagnation, the streaks, wrinkles and uneven pattern transfer are not generated on the surface of the molding product. Accordingly, a molding product having beautiful gloss and clear transparency can be obtained.
Incidentally, another blow-molding technique for sealing much gas such as air in the parison in advance (Japanese Patent Publication No. Hei 06-55423, Japanese Patent Laid-Open Publication No. Hei 07-214649) has been proposed. However, in such technique, the surface of the parison and the inner surface of the mold are brought into contact in clamping the mold by the much amount of gas sealed in beforehand. Accordingly, the parison can be excessively inflated to be fat, so that the parison can go beyond the mold or explode in accordance with clamping the mold. In the present invention, since no excessive gas is sealed in clamping the mold, the above problem can be avoided.
Further technique using a frame like the parison shutter of the present invention (Japanese Patent Number 3012837) has also been proposed. However, since the parison is not shut (sealed) in the technique, a pressing effect toward the inner surface of the mold by the inner pressure of the gas is difficult to be obtained and improvement in moldability as in the present invention cannot be obtained. Further, a part of the parison is likely to stick out.
In the method of the present invention, the step of pre-blowing for blowing air into the parison before shutting the parison by the parison shutter may preferably be provided.
In the present invention, the force for abutting (pressing) the parison onto the mold is generated by the difference between the pressure of the air (gas) in the parison in clamping the mold (internal pressure of the parison) and the pressure of the air (gas) in the space between the parison and the mold. When the parison is kept, i.e., enclosed, in the closed space by the parison shutter, since the internal pressure of the parison can be adjusted by adjusting the air amount in the parison, the internal pressure of the parison, i.e. the force for bringing the parison into contact with the die design surface can be increased by increasing the air amount inside the parison by conducting pre-blowing before shutting the parison, so that the parison can be smoothly into contact with all of the die design surface in clamping the mold even when the die design surface has complicated configuration.
At this time, the air feeder for conducting the pre-blowing may be projected from any direction relative to the parison, however, the air feeder may preferably projected from the side or vertical direction of the parison.
In the present invention, when the parison is shut by the parison shutter, a space surrounded by an outer surface of the parison and an inner surface of the mold may preferably be arranged as a closed space and the mold may preferably be closed after decompressing the closed space (for instance, below the atmospheric pressure, and negative pressure relative to the pressure inside the parison).
Specifically, since the parison shutter shuts the parison, the closed space having no air circulation with the outside is formed in the area surrounded by the outer surface of the parison and the die design surface. And, for instance, by creating negative pressure by, for instance, discharging the air (any gas) enclosed in the closed space, the pressure difference between the inside of the parison and the closed space can be increased. As a result, the force for contacting (forcing) the parison against the die design surface is generated. Accordingly, since the pressure difference between the air pressure inside the parison and the air pressure within the closed space can be increased as compared with a case where only the parison shutter is used and the closed space is not arranged in negative pressure, the parison can securely touch the die design surface in clamping the die without pre-blowing, so that a molding product having further improved appearance can be obtained.
Further, by combining the pre-blowing and the negative pressure, the pressure difference between the air pressure inside the parison and the air pressure in the closed space can be further increased. In this case, the parison can be in contact with the die design surface even with the die design surface with more complicated configuration.
Since a blow-molded product obtained by the production method of the present invention shows superior appearance without streaks and wrinkles even by transparently coating or not coating the surface after ejecting the parison from the inside of the mold, troublesome work for color painting can be omitted.
A blow-molded product according to the present invention is characterized by being molded by the respective blow-molding methods.
Specifically, a blow-molded product according to the present invention is a molding product molded by introducing a parison extruded from a die into a mold, the mold being providing with a parison shutter for shutting the parison to keep the parison from inflating out of the mold, the molding product being produced by: opening the mold; introducing the parison into the mold; shutting the parison by the parison shutter without bringing the parison into contact with the inner surface of the mold; and closing the mold to compress the parison to bring the parison into contact with the inner surface of the mold.
In the blow-molded product of the present invention, the parison as a pre-solidified member of the molding product is shut by the parison shutter in advance to clamping the mold between the opposing molds without being inflated out of the mold. Accordingly, in the subsequent mold clamping step, the parison touches the design surface continuously from the convex portion (pre-contact portion) to the concave portion (blow-up portion). In other words, molding failure caused when a part of the parison goes beyond the mold and the air pressure inside the parison is decreased so that the parison does not touch the die design surface can be avoided, thereby securely and rapidly bringing the parison into contact with the die design surface. Accordingly, the parison can be uniformly contracted in cooling after the contact, thus avoiding molding failure such as streaks, wrinkles and uneven pattern transfer to obtain a blow-molded product having a surface with beautiful gloss and clear transparency.
A blow-molding mold according to the present invention is for accommodating and blow-molding a parison extruded from a die, the mold being characterized in having a parison shutter for shutting the parison extruded from the die to keep the parison from inflating out of the mold.
In the present invention, the parison cylindrically extruded from the die into the mold is shut by the parison shutter provided to the mold before clamping the mold without inflating toward the outside of the mold while being accommodated within a space surrounded by the die design surface and the parison shutter.
When the mold is clamped in the above condition, the parison initially touches the convex portion (pre-contact portion) projecting toward the parison and is pressed, so that the air pressure inside the parison is raised. Accordingly, after the parison touches the convex portion, the parison continuously touches the concave portion of deeply carved mold (blow-up portion) without intermission. In other words, since the parison continuously touches the mold without stagnation and the whole parison is uniformly cooled, the streaks, wrinkles and uneven pattern transfer are not generated on the surface of the molding product. Accordingly a molding product having beautiful gloss and clear transparency can be obtained.
The parison shutter of the present invention may preferably include a frame body respectively provided around a periphery of opposing molds and advanceable and retractable from one mold to the other mold, the frame body being abutted to shut the parison within a closed space isolated from an outside of the mold by the frame body and the mold.
In the above arrangement of the present invention, since the parison shutter for shutting the parison is provided around the mold, in other words, since the space formed by the parison shutter and the mold has no gap for allowing the parison to inflate out of the mold, the enclosed parison can be securely accommodated between the opposing molds. Accordingly, the inflation toward the outside of the mold in clamping the mold can be securely restrained, so that the parison can securely touch the die design surface.
At this time, the movement of the frame body for shutting the parison is simple advancement and retraction. Further, since the frame bodies are respectively attached to the respective molds, the stroke amount of the frame body (enclosure amount) can be restrained within a range a little larger than the radius of the parison. Accordingly, since the drive mechanism for actuating the frame body can be made simple, the arrangement of the parison shutter, and as a result, the arrangement of the blow-molding mold itself can be simplified, thus facilitating the production of the blow-molding mold.
In the blow-molding mold of the present invention, the blow-molding mold may preferably has a design mold having a die design surface and an opposing mold opposing the design mold, the parison shutter being advanceable and retractable from the opposing mold to the design mold, and the parison shutter having a frame body formed on a land portion around the design mold for the parison to be pressed.
According to the present invention, since the parison shutter is constructed by a simple frame provided on only one of the die, the number of the provided frame is one in contrast to providing both opposing molds with the frames, thereby reducing cost for producing the parison shutter and, in the end, the cost of the molding product.
When the blow-molding mold having the frame on one side, following blow-molding method may preferably used.
Specifically, the parison may be pressed against the land portion around the design mold, and, after or simultaneously with decompressing a closed space surrounded by an outer surface of the parison and the die design surface of the design mold, the design mold and the opposing mold may be closed.
According to the above arrangement, the parison can be securely pressed against the design surface, thus obtaining excellent moldability while simplifying the blow-molding die.